"How accurate is a 3D photogrammetry model without Ground Control Points?" This question is often asked by our customers but there was never a definitive answer, until now.
But first some background on Ground Control Points (GCP) and why they are used. GCPs are currently used by SiteSee to ensure both accuracy scale and orientation of 3D models of cell towers. GCPs are static markers that accurately identify a point on the earth's surface. These markers typically use GPS with a correction network to provide very accurate static positioning. In the photogrammetry processing these markers are identified in the imagery and given an accurate 3D position. The bundle adjustment is now constrained by these fixed points, allowing for a more accurate 3D model.
The 3D model characteristics of dimension scale and angle are important for the SiteSee automated object recognition system. The scale determines the size of each of the three dimensions. For cell towers the most important dimension is along the vertical (z axis), as the height of the cell tower equipment is one of the most important measurements. The orientation of the cell tower allows for accurate absolute bearings and tilts of equipment.
This accuracy comparison will use a single acquisition data set of a cell tower. The only difference between the photogrammetry results is one processed with GCP, the other without, allowing for a simple comparison.
This experiment is not meant to be an exhaustive exercise in surveying, but rather a short, sharp determination of the suitability of the SiteSee photogrammetry model's accuracy without GCP compared to using GCP. It will measure points between two identical elements of a Telco lattice tower on two 3D models prepared by SiteSee using photogrammetry, UAV, and with/without GCP.
The two model elements being measured are: a) underside of a gantry to b) the underside of the steel resting on the footing. Although there are three dimensions of each point, this experiment is concerned with the major dimension of the telco tower — the vertical or z dimension.
Target: Telco Tower, Pullenvale Brisbane Australia
Photogrammetry (GCP): DJI Phantom 4Pro, 5× Propeller Aeropoints, ContextCapture v4.4.8.561
Photogrammetry (No GCP): DJI Phantom 4Pro, ContextCapture v4.4.8.561
Measurement software: CloudCompare V2.91 64bit
Acquisition Background
The tower was flown with a DJI Phantom P4, using the Drone Harmony autopilot app. Propeller Aeropoints were used for Ground Control Points (GCP). The flight plan follows SiteSee's proprietary vertical structure flight plan. Approximately 1,500 images were captured. ContextCapture was used to perform the photogrammetry. No manual tie points were used. The resulting model was a mesh in OBJ format with texture.
Results
Scale Results
The scale accuracy results demonstrate a difference of -0.855m over 50m between the GCP and No GCP models. This figure seems surprisingly low. However, when considering that the majority of cell tower equipment is located at the top of the tower, the error in the equipment is significant.
Orientation Results
To determine relative orientation between the models, points along an external member were used. Points were selected on the outside of the member, one near the base and the other near the top. This allows comparison of the orientation of the same external member between models. Using the dot product method, the smallest angle between the vectors describing the external member was calculated. The result is a surprisingly small angular difference.
Wrap Up
The comparison of a photogrammetry model reconstruction using GCP vs NoGCP has produced reasonable accuracy based on measures of scale and orientation. The scale difference will become an issue for any accurate vertical measurement such as Telco towers. The orientation difference seems to be far less and may not be that significant for some applications.
However, for automated object recognition these accuracy differences mean that GCP will still be used for data acquisition and processing, to ensure accurate "As-built" of Telco assets.